Many cellular communication systems consist of a combination of different types of systems and protocols. Likewise, many of the devices used on these networks, such as cellular telephones and handheld personal data assistants, are designed to be multi-mode devices, i.e. will operate on multiple networks. Ideally, there should be no degradation or otherwise negative indication that a device is on one network or another to an end user. However, certain factors do affect performance on different networks. Battery life may be degraded on one system in comparison to another as the hardware required to operate in that particular mode may consume more energy than in another mode. Also, cost may be significantly different from one system to another and the subscriber may pay a premium to be able to use a particular network.
Many locations, such as workplaces and universities, are beginning to deploy wireless local area networks (WLANs) within a particular site or building. For these systems, devices having multi-mode operation will operate on the WLAN while within the building or site and switch over to a macro or wide area network (WAN), such as a GSM (Global System for Mobile communications), CDMA (Code Division Multiple Access), WCDMA (Wideband Code Division Multiple Access), iDEN (integrated Digital Enhanced Network) or TDMA (Time Division Multi-Access) cellular system, when the user leaves the coverage area of the WLAN. While on the WLAN system, calls are typically made using Voice-over-IP and provides a great cost savings for both the user and the cellular system.
These multi-mode devices, which are capable of operating on WLAN and WAN systems, can consume a significant amount of battery power looking for service on the WAN while camped on the WLAN. Some of this is purely wasted energy when WLAN coverage is good, and given that WLAN is the preferred connection, WAN background scans are unnecessary. However, in the event that the device knows nothing about the WAN service, it must go on the assumption that it could take minutes to find an acceptable service and therefore, should begin searching immediately. To complicate matters further, quite often more than one service provider is available on the WAN side requiring the device to search over all potential frequencies and supported Radio Access Technologies (RAT's) and not only find the channel that the preferred RAT is on but also the preferred service provider. Even if a service is found, that operator may not support the desired feature set for an application of interest requiring the device to move on and continue looking, or settle for what can be provided. This not only wastes time, but also uses extra battery life.
In order for this multi-mode system to be effective, it is desirable to have a seamless integration between systems. It is critical to be able to transfer a call in progress on one system over to a different system without a noticeable consequence to the end user, such as a lost call, an unnecessary charge, significant reduction in battery life, etc. In most instances, the burden to decide when to hand over to the other system is placed upon the actual subscriber device.
Several techniques exist for making this decision. The first, most obvious method is a “brute force” method as illustrated in FIG. 1. A typical cellular communications system consisting of overlapping WAN cells 102, with a concentration of overlapping WLAN cells 104 within a building 108 is shown. Currently the handover for calls between WLAN coverage and WAN coverage is based on relative signal strengths. However, due to the non-uniform nature of coverage caused by building structure, obstructions etc., making an accurate reliable decision to initiate the handover of the call from one system to another is difficult and error-prone. Scanning both systems costs battery life on the Mobile Device. Furthermore, the presence of any WLAN coverage holes inside the building complicates this decision making process and may result in short WAN calls when the user walks through a WLAN coverage hole, say in a stairwell for example. While within the area of WLAN coverage, the mobile subscriber device 106 is continually running the hardware and software stack for one system, and performing background checks for the other system. This insures that whenever the user leaves coverage of the WLAN system, the call is handed over to the WAN system with no noticeable interruption to the user. The problem with this method is that there is significant detrimental effect on battery life because the device must operate both sets of hardware simultaneously. Moreover, there is actually an inherent degradation in battery life of these devices in comparison to single-mode devices due to the additional power required by the WLAN chipsets. While there may be a cost savings for calls made on the WLAN system, this advantage is offset by the loss in battery life.
A prior art system that uses a more intelligent handover method is shown in FIG. 2. In this example, border cells 210 are placed near the entry and exit doors 212 of the building 108. A border cell 210 generally is served by a WLAN access point (AP) in the vicinity of entry and exit points 212. The cell has been designated as a border cell 210 and a border cell AP transmits information to the subscriber unit (SU) 106 that identifies the cell as a border cell 210. The AP would typically transmit such information in the 802.11 beacon frames and in the probe responses. Normally, before a user is leaving the WLAN coverage area 104, the subscriber unit first detects a border cell 210. The SU 106 starts the cellular stack and begins to register with the cellular network 102 upon detecting the border cell 210 information so that the call is handed over in an adequate timeframe. By the time the SU 106 is out of range of the WLAN coverage area 104, the call should be transferred to the WAN 102.
When a user is leaving the WLAN coverage 104, the decision to hand over from the WLAN 104 to the macro network 102 needs to be made as early as possible to avoid dropping calls due to the rapid falloff of the indoor WLAN coverage at the outside of the building 108. Additionally, the handover decision must not be made prematurely to avoid the service cost associated with handing over a subscriber to the macro network 102 when the SU 106 does not actually leave the WLAN coverage area 104. Similarly, users that are in a call while exiting the building or taking breaks near the entryways (e.g. smoking near exits in WLAN coverage areas 104), are unfortunately mistakenly handed over to the WAN 102.